What is the primary method for achieving capacity control of a centrifugal compressor?

Achieving capacity control of a centrifugal compressor primarily involves adjusting the inlet impeller pre-rotational vanes. This method allows for modulation of the flow characteristics of the refrigerant entering the compressor. By varying the angle of the pre-rotational vanes, the compressor can effectively change the velocity and direction of the incoming refrigerant, optimizing performance according to the demand for cooling or heating. This adjustment directly influences the volume flow rate and pressure ratio, thereby providing a robust way to control capacity efficiently.

In contrast, adjusting the discharge valve may influence the system's pressure but does not directly affect the flow into the compressor. Changing the refrigerant type can have implications on the system's overall efficiency and operational characteristics, but it does not offer a flexible or immediate method for capacity control in real-time operations. Modulating the suction pressure affects the performance of the compressor, but it is not as direct or effective as adjusting the inlet vanes for controlling capacity under varying load conditions.